Amplification system for crystal pickups and microphones



Nav. 5, 1940.

J. H. Risas 2,220,281

Filed ot. 5, 195s Il MH Patented Nov. 5, 1940 UNITED STATES AMPLIFICATION SYSTEM FOR CRYSTAL PICKUPS AND\MICROPHONES John H. Riggs, North Tonawanda, N. Y., assigner to The Rudolph Wurlitzer Company, Cincinnati, Ohio, a corporation Application October 5,

9 Claimsl This invention relates to an amplication system for crystal pickups and microphones. It is well known that when a piezo-electric crystal such as the Rochelle salt is used as a device for transforming mechanical motion or pressure into electrical energy, the output voltage of the crystal varies with the temperature. This Variation in output of the crystal is ordinarily greater at the lower audio-frequencies, the eiect being readily noticeable to a person listening to music reproduced from a phonograph record by a piezoelectric pickup or relayed through a piezo-electric microphone. A listener will notice a diminishing of base notes and a falling o' in volume 5 level as the temperature at the crystal rises above about 75 F.

One object of the present invention is to provide an amplification system for piezo-electric pickups or microphones in which this objectionable variation in volume is substantially eliminated.

Another object of the invention is generally to improve the amplication of the output of crystal pickups and microphones.

Other objects and features of the invention will be understood from the accompanying drawing and the following description and claims:

The drawing is a diagram of electrical connecnections showing the invention applied to a crystal pickup.

In the drawing, the reference character Ill indicates a crystal pickup of any well known form by means of which mechanical movement such as the vibration of a stylus following the groove of a phonograph record is transformed into pressure exerted upon a piezo-electric crystal. The variations in pressure cause a correspondingly varying electromotive force to be generated within the crystal and to be impressed upon conductors I-I and I2 connected to appropriate portions of the crystal. A load impedance I3 is connectved across the conductors Il and I2 and passes an electric current caused by the electromotive force generated in the crystal. 'I'he conductor I2 is grounded as shown and the conductor II is connected to a blocking condenser I4, the opposite terminal of which is connected to the control grid I5 of athermiom'c tube I6.

The tube I6 is of a type known commercially as 6L7 or "6L'lG and is classified as a pentigrid mixer ampliiier. It is provided with a cathode I'I which may be heated by a filament I 8 supplied with electric current from any suitable source, not shown. The tube also has the usual anode I9, a suppresser grid 28 connected with 1938, Serial No. 233,423

the cathode in the usual manner and three screen grids 2|, 22 and 23. The grids 2I and 22 are connected together and are maintained at a substantially uniform potential with respect to the cathode. The grid 23 is placed between the grids 2| and 22 and is supplied with a varying potential. The ratio of amplication or gain of the tube may be controlled by varying the potential of the grid 23, an increase of said potential increasing the gain of the tube and a decrease in 10 said potential decreasing said gain.

The necessary potentials for the various eley ments of the amplication circuits may be provided as shown in the diagram by batteries 24, 25, 26 and 2l or by other'suitable arrangement. l5 By way of example, desirable potentials are indicated by numerals in the drawing but need not be strictly followed in securing the benefits of the invention. The control grid I5 is connected through the usual grid leak resistance 28 with a 20 point on the `potential supply system at plus 40 volts. The cathode is connected by a conductor 28 with a point maintained at plus 50 volts. The grids 2l and 22 are connected by a conducto-1'. 30 with a point maintained at plus volts. The 25 anode I9 is connected through a loading resistance 3I to a point maintained at plus 300 volts and is also connected to a condenser 32, the opposite terminal of which is connected to one of a pair of output terminals 33, the second one of 30 which is grounded. Any suitable apparatus for further amplification may be connected to the output terminals or they may be connected directly to a speaker if the gain of the tube I6 is sufcient to operate a speaker. I 35 The grid 23 is connected by a conductor 34 to a point intermediate between a pair of resistance elements 35 and 36. The opposite terminal of the resistance 36 isgrounded and the opposite terminal of the resistance 35 is connected 40 through a resistance 31 to the conductor 30. By this connection, a current constantly flows from the conductor 30 through resistances 31, 35 and 36 to the ground and the potential impressed upon the grid 23 depends upon the voltage drop 45 er frequencies. Thus, an increase of temperature reduces the volume of the low frequency sounds and has little or no effect on the volume of the high frequency sounds. According to the present invention, the load impedance I3 is of the order of 25,000 ohms. It has been found after extensive experiments, that this relatively low impedance produces a much more marked telnperature effect at the higher frequencies than does the high impedance heretofore in use. There is also some increase in the etemperature effect at the low frequencies and of course the low impedance produces a general lowering of the level of the voltage impressed on the condenser I4. However, the net effect of the low impedance is to make the temperature effect on the crystal substantially uniform through the range of useful frequencies and thus to eliminate the frequency discrimination heretofore found in the temperature effects of piezo-electric pickups.

In order to maintain a substantially constant level of output volume irrespective of temperature changes, the resistances 35 and 36 are made of materials having different temperature resistance coefficients and are so placed that they are affected by the same room temperatures which affect the crystal. These coefficients are so chosen that an increase in room temperature increases the resistance of the element 36 in a much greater amount than the resistance of the element 35 may be increased. The increase in room temperature therefore increases the potential impressed on the grid 23 and thus increases the gain of the tube I6 to provide greater amplification. By this means, the amplification of the tube increases as the output of the crystal decreases due to a temperature rise. By a suitable choice of the resistances 35 and 36 and of the materials from which they are made, the increase in gain may be made to compensate for the loss in crystal output and to maintain a uniform volume level with a high degree of accuracy.

The foregoing specification describes thek invention in one of its preferred forms. Changes in the details thereof may be made by those skilled in the art Without departing from the scope of the invention as defined by the appended claims. For example, the invention is equally applicable to crystal microphones or to any other apparatus in which a pressure responsive crystal is used to transform mechanical motion or pressure into electrical energy. It is also possiblejto use other temperature responsive means for controlling the gain of the tube than the specific means shown herein, although the latter means in itself constitutes an important feature of the invention.

The invention claimed lis:

1. A method of amplifying the output of piezo-electric crystal by means of a vacuum tube amplifier including the steps of feeding the output of the crystal to a control electrode of the tube, and automatically varying the gain of the tube in response to temperature changes to compensate for changes in output of the crystal caused by such temperature changes.

2. A method of amplifying the output of a piezo-electric crystal byf'means of a vacuum tube .amplifier including the steps of modifying the output of the crystal to compensate for frequency discrimination in the crystal caused by temperature changes, -feeding the modified output of said crystal to a control electrode of the tube, and automatically varying the gain of the tube in response to temperature changes to maintain a substantially constant output level irrespective of the effect of such changes on the output of the crystal,

3. In an amplifier for the output of a piezoelectric crystal, the combination of a vacuum tube having a heated cathode, an anode, a control electrode and an auxiliary electrode adapted to control the gain of the tube by means of varying potentials impressed thereon, a circuit receiving the output of said crystal and connected to the control electrode of the tube, an output circuit connected to said anode, and a circuit connected to said auxiliary electrode and including a temperature responsive element adapted to vary the potential impressed on said auxiliary electrode inresponse to variations in the temperature affecting said crystal, whereby the gain of the tube may be controlled to compensate automatically for variations in strength of crystal output caused by such temperature variations.

4. In an amplifier for the output of a piezoelectric crystal, the combination of a Vacuum tube having a heated cathode, an anode, a control electrode and an auxiliary electrode adapted to control the gain of the tube by means of varying potentials impressed thereon, a circuit receiving the output of said crystal and connected to the control electrode of the tube, said circuit including a loading element of sufficiently low impedance to eliminate frequency discriminations of said crystal caused by variations of the temperature aecting said crystal, an output circuit connected to said anode, and a circuit connected `may be controlled to compensate automatically for variations in strength of crystal output caused by such temperature variations.

5. In an amplifier for the output of a piezoelectric crystal, the combination of a vacuum tube having a heated cathode, an anode, a control electrode and an auxiliary electrode so placed with respect to the elements of the tube Yas to increase the gain of the tube when the potential impressed thereon is increased and to decrease thelsame when said potential is decreased, a circuit receiving the output of said crystal and connected to the control electrode of the tube, an output circuit connected to said anode, and a circuit connected to said auxiliary electrode and including a temperature responsive element adapted to increase the potential impressed on v said auxiliary electrode in response to an increase in the temperature aecting said crystal and to decrease said potential injresponse to a decrease in said temperature, whereby the gain of the tube may be controlled to compensate automatically for Variations in strength of crystal output caused by such temperature variations.

6. In an amplifier for the output of a piezoelectric crystal, the combination of a vacuum tube having a heated cathode, an anode, a conmeans for impressing a substantially constant voltage across said resistance elements and said elefnents being -so placed as to be affected by the the output of said crystal, and temperature con.

trolled means responsive to the temperature af-n fecting said crystal for automatically varying the ratio of amplication of said ampliier to compensate for changes in the outputstrength of the crystalvcaused by variations in said temperature.

8. In an amplier for the output of a piezoon said auxiliary electrodeelectric crystal, the combination of a load circuit connected to said crystal and having a. suiilciently y low impedance to insuresubstantially uniform temperature effects over the rangeofuseful frequencies, a vacuum tube amplier having an input circuit connected to said load circuit, and

temperature controlled meansresponsive to thetemperature .affecting said crystal for automatically varying the ratio o! amplification of said amplier to compensate for changes in the output strength of the crystal causedby variations in said temperature.

9. In an amplifier for the output o'f a piezoelectric crystal, the combinationv of a load circuit connected to said crystal and having an impedance of the order of 25,000 ohms,. a vacuum tube ampliflerhaving an input circuit connected tov said load circuit, and temperature controlled lmeans responsive to the temperature affecting the ratio of amplification 'of said ampliiier` to compensate for changes in the output strength oithe crystal caused by variations in said temperature.

' JOHN H. RIGGS. 

